1. Field of the Invention
This invention relates to improved catalysts for the selective dehydrogenation of dialkyl aromatic hydrocarbons to produce alkyl vinyl aromatic hydrocarbons, more particularly to catalysts for the production of para-methylstyrene (PMS) via the dehydrogenation of para-ethyltoluene (PET).
2. The Prior Art
The vinyl benzenes play a particularly important role in the preparation of synthetic plastics and resins. The polymerization of styrenes, for example, to produce polystyrene resins is well known.
Styrene and styrene derivatives are typically produced from ethyl benzene materials by dehydrogenation over solid catalysts in the presence of steam, and at temperatures ranging from 500.degree. to 700.degree. C. The catalysts found to be the most effective for this process are those which are based on potassium oxide (carbonate) promoted, chromium oxide stabilized, iron oxide material. Considerable research has been directed toward attempts to improve the activity and selectivity of this class of catalysts. Any improvement which results in either increasing the selectivity (moles of desired product per mole of reactant reacted) or the conversion (moles of reactant reacted per mole of starting material) without lowering the other is economically attractive since the result is that the yield (moles of desired product produced per mole of reactant) of the product has been increased. Any increase in the numerical value of the yield results in a more efficient operation with more reactant being converted into the desired product. In commercial operations, many of which produce millions of pounds of product per year, a trade-off is frequently effected between selectivity and conversion. An increase of only 1 or 2 percentage points in the selectivity can result in a substantial savings of starting materials. An increase in conversion can substantially reduce capital expenditure and energy consumption. The trade-off may vary depending on raw materials costs, energy costs, and the age of the plant.
Attempts have been made to improve the conversion effectiveness and selectivity of iron oxide type dehydrogenation catalysts for use in various alkylaromatic dehydrogenation reactions. Riesser; U.S. Pat. No. 4,152,300; issued May 1, 1979, for example, discloses that an improvement in ethylbenzene dehydrogenation catalyts selectivity can be realized by incorporating small amounts of certain metal oxide materials into dehydrogenation catalyst compositions comprising mixtures of iron oxide, potassium oxide, vanadium oxide and, optionally, chromium oxide.
Courty; U.S. Pat. No. 4,134,858; issued Jan. 19, 1979, discloses an iron oxide based dehydrogenation catalyst containing particular amounts of clay to improve the conversion, selectivity and yield of styrene and divinylbenzenes produced by dehydrogenation of ethyl- or diethylbenzene. This U.S. Pat. No. 4,134,858 also notes that oxides of copper, vanadium, zinc, manganese, magnesium, nickel, cobalt, bismuth, tin and antimony can be added to the disclosed dehydrogenation catalysts.
One factor not considered by the foregoing references and a factor which can affect the economical production of certain styrene derivatives such as p-methylstyrene via the steam dehydrogenation of p-ethyltoluene over catalysts of the foregoing type is the phenomenon of "popcorn" polymer formation. Popcorn polymers are those solid polymeric materials which are popcorn-like in appearance and which can form and build up at the relatively cooler exit port of the dehydrogenation reaction reactor vessel and in the condenser and other parts of the cooling train used to recover p-methylstyrenic dehydrogenation products. Popcorn polymer formation does not generally occur when ethylbenzene is dehydrogenated but can be a significant problem during dehydrogenation of para-ethyltoluene. While various inhibitors can be added to the dehydrogenation reactor effluent to minimize popcorn polymer formation, the eventual buildup of such materials can necessitate shutdown of the dehydrogenation reaction equipment to remove the accumulated popcorn polymer. Obviously, dehydrogenation catalysts which reduce the inherent tendency of popcorn polymers to form in the dehydrogenation reaction effluent would be economically attractive for commercial scale production of styrenic materials such as p-methylstyrene.
In view of the foregoing considerations, there is clearly a continuing need to formulate improved steam regenerative dehydrogenation catalysts suitable for production of particular styrenic materials from methyl-substituted ethylbenzene materials, e.g., selective production of p-methylstyrene from p-ethyltoluene. Such catalysts are those which can be used to realize improved, e.g., sustained conversion, selectivity, and/or yield in the dehydrogenation of particular alkylaromatic materials such as p-ethyltoluene with reduced popcorn polymer formation.
Accordingly, it is an object of the present invention to provide an improved iron oxide based dehydrogenation catalyst especially useful for the dehydrogenation of para-ethyltoluene to produce para-methylstyrene.
It is a further object of the present invention to provide a para-ethyltoluene dehydrogenation process employing a catalyst which produces either a significant increase in para-ethyltoluene conversion with little or no corresponding drop in para-methylstyrene selectivity or a significant increase in para-methylstyrene selectivity with little or no corresponding drop in para-ethyltoluene conversion.
It is a further object to provide such a dehydrogenation process employing catalysts which have a reduced tendency to promote the undesirable formation of "popcorn" polymer.
These and other objectives can be achieved by means of the invention described and claimed herein.